Process of hardening varnish coatings on leather



PRGCESS 'OF HARDENING VARNISH COATINGS N LEATHER Ernst Demme, Weinheim an der Bergstrasse, Germany No Drawing. Continuation of applications Serial No. 222,745, April 24, 1951, and Serial No. 258,299, November 26, 1951. This application October 26, 1954, Serial No. 464,889

5 Claims. (Cl. 117-62) This invention relates to improvements in-the production of patent leather and is a continuation and consolidation of my co-pending applications Serial Nos. 222,745, filed 'April 24, 1951, and 258,299, filed November 26, 1951, now both abandoned.

Patent leather is generally produced in accordance with the hot varnish process. This process is described in detail in Stather, Gerbereichemie and Gerbereitechnologie, beginning on page 539. In accordance with this process, linseed oil varnish is, for example, a thinly liquid boiled linseed oil which is mixed with thinner such as gasoline or turpentine and which, if desired, may contain dye pigments, is applied to bottomed and buffed leather stretched on a frame. The term leather as used herein and in the claims is intended to designate any natural or artificial leather. The leather is stretched on a frame and the coat of varnish is then dried for about twelve hours at 50-55 C., preferably in a horizontal position, in a japanning or varnishing stove or oven.

The varnish coating of the dried leather, after this oven drying, is, however, always sticky and tacky and must be subjected to a further treatment in order to surface harden the same and remove the tackiness.

Formerly, this additional drying and hardening was carried out in the air by exposing the varnish to sunlight. More recently, this after-treatment was efiecte'd by irradiating the varnished leather with artificial ultra violet light as, for example, by irradiating with mercury quartz lamps for about two hours.

The irradiation with the ultra violet light is, however, relatively diliicult and expensive and not completely satisfactory. The ultra violet light equipment is relatively costly to obtain and maintain. It is often impossible to avoid over-radiation which causes excessive hardening of the varnish coat so that the same becomes brittle and will crack if subjected to sharp bending or folding.

It has been suggested to harden varnish coatings by treating the same with liquid or solid polyisocyanate, including diisocyanates, or by incorporating the isocyanate in the varnish coating.

This treatment, however, has not proved satisfactory for removing the tackiness in connection with patent leathers. The liquid or solid isocyanate would cause spottiness of the varnish coating and cause a setting reaction throughout the entire thickness of the coating so that the same would become brittle and crack if the leather were sharply bent or folded.

One object of this invention is a method for surface hardening and removing the tackiness of the oven dried patent leather from the hot varnish process without the above mentioned difliculties. This, and still further objects, will become apparent from the following description:

In accordance with the invention, the oven dried patent leather containing the linseed oil varnish coating which is still tacky is contacted with an isocyanate vapor. The isocyanate vapors within about 3-5 hours will produce an nited States Patent 'ice entirely unobjectionable drying and hardening of the surface of the varnish with a removal of all tackiness and without any detrimental influence on the varnish layer or base of the leather.

Due to the nature of the diisocyanate vapors, the treatment is carried out in gastight compartments in order to avoid health hazards. The leather is first introduced into these compartments, which are closed and charged with the isocyanate vapors by means of suitable apparatus, such as Vaporizers, pumps, or the like. After the termination of the hardening, the vapors are withdrawn from the chambers by exhaust fans or the like, while permitting fresh air to ilow in.

Any diisocyanate which may be vaporized without decomposition may be used for the treatment in accordance with the invention. It is preferable to use diisocyanates having a relatively high vapor pressure as, for example, hexamethylene diisocyanate, 1 methyl-2-4 phenylene diisocyanate (melting point 21.6" C.) or 1 methyl-2-6 phenylene diisocyanate (M. P. 83 C.) or mixtures of the same.

The temperature and pressure at which the contacting with the diisocyanate vapors is elfected is not at all critical and may vary over wide ranges, so long as the diisocyanate remains in vapor form. The contacting may be effected, for example, at normal temperatures and pressures.

In accordance with a preferred embodiment of the invention, the oven dried varnish coated patent leatheris irradiated with ultra violet light before the treatment with the diisocyanate vapors. The irradiation with the ultra violet light may be effected in any manner conventionally used for the hardening of the patent leathers, but should be interrupted before the surface coating is completely hardened. The final hardening is then effected by contacting with the diisocyanate vapors, as described above.

By employing the irradiation with the ultra violet light in conjunction with the treatment with the diisocyanate vapors, certain very definite advantages are secured. For example, the irradiation step does not have to be prolonged or carried out to the extreme limit, thus avoiding the danger of overhardening the varnish layer or rendering it excessively brittle. In the succeeding treatment with the diisocyanate vapors, all traces of the tackiness of the varnish layer are eliminated without any danger of injurious or destructive side effects.

By treating the oven dried linseed oil varnish coated leather with the ultra violet light, the subsequent treat ment with the isocyanate vapors is considerably shortened. Without prior ultra violet irradiation, the isocyanate treatment generally takes from 3-5 hours. With the prior irradiation, it is possible to reduce the duration of this isocyanate vapor treatment to as little as about one half-hour.

It has further been found that the total treatment time, when using the combination of ultra violet irradiation and diisocyanate treatment, is considerably shorter than the total treatment time whenusing either ultra violet light irradiation or treatment with diisocyanate vapors alone.

Thus, for example, when using quartz mercury, lamps for the ultra violet irradiation, it generally tool: from about 1%2 hours to completely harden the varnish coating and completely eliminate the tackiness. When the same source of irradiation in'combination' with the after diisocyanate vapor treatment, the irradiation period may be reduced to a period of about /z-l hour. Thus, total treatment time, including the period of ultra violet irradiation and treatment with the vapors of the diisocyanate, is usually only about 1 hour, as compared with about l%.-2. hours. treatment with. ultra violet. irradiation alone, or 3-5 hours with diisocyanate vapors alone.

The present invention offers a great advantage in allowing the removal of the surface tackiness' of the patent leather in. a simple. and. reliable manner, without having anyv undesirable. or detrimental. secondary effects. By treatment with the diisocyanate vapors-in accordance with the. invention, an. unobjectionably hardened surface skin is formed on thevarnish, while the under'varnish: layer is not detrimentally influenced in the hardening process. The under varnish layer, therefore, remains soft. and flexible. serviceability and. durability of. the patent leather, which becomes. particularly important at lower temperatures. Iffthevarnislrwouldlbe'hardened through itsentire thickness, as in the case of treatment with liquid or solid diisiocyanates, it. would become brittle and, when used, would soon crack. and" becomev irregular. When using the diisocyanate vapors. for the surface hardening, in

accordance with the. invention, the hardening process may be-carried out in an. exactly controlled manner and interrupted, for example, at any desired. moment. The

properties of the varnish coating may beextensively influenced and, for example, lustrous or dull varnish coatings may beproduced.

Thefollowing examples. are given by way of illustra-- tion, and not limitation:

Example 1 10 hides of natural leather and 10 pieces of artificial leather are bottomed. and buffed in the known manner. The leather pieces are each about 30 x 30 cm. in size. Linseed oil varnish is prepared by boiling linseed oil until it is thinly liquid and thereafter thinning it with turpentine. This lacquer is applied to the leather pieces in the conventional manner. The leather pieces are. then stretched on frames. and dried on the frames ina japanning or varnish oven for about 12 hours at 50-5 C. The leather pieces arethen removed from the oven. The varnished surface of the leather is still tacky. The production of the patent leather up to this point is identical with the manner described by Stather in Gerbereichemie und Gerbereitechnologie, pages 539 supra.

The'oven dried leather pieces still on the frames are then placed in a gastight compartment having a volume of cubic meters. An evaporating basin on a heating plate is positioned in the compartment in a space about 50 cm. below the frames. A glass tube leads downwardly through the wall of the compartment to. the evaporating. basin. The heating plate is heated and liquid isocyanate is introduced through the glass tube so the same falls into the evaporating. basin, rop by drop. The compare ment is provided with an observation window and the interior is illuminated so that the supply of the isocyanate may be regulated. The isocyanate vaporizes and a fan placed at the bottom of the compartment distributes the vapors. During the treatment, 100 grams of theis0cyanate are used, and the treatment is continued for about 3 hours, at the end of which time the isocyanate vapors are exhausted.

The varnished leather in the form of patent leather is thenv removed from the compartment. The varnish coating is extremely glossy and regular and may be creased or bent sharply without any cracking or distortion occurring.

3 batches of leather are treated. in this manner, with hexamethylene diisocyanate, 1 methyl-24 phenylene diisocyanate, and 1 methyl2-6 phenylene diisocyanaterespectively. The same excellent results are obtained in each case.

Example 2 2.0 skins having a total area of 270 square. feetwere coated with linseed oil varnish and oven dried in the This. isof prime importance for the.

4 manner described in Example 1. After, the oven drying, the varnish coating was still tacky. The skins were then irradiated for minutes with ultra violet light by itradiating with individual 300 watt quartz mercury lamps positioned at a distance of 40. cm..from the skins. After the treatment with the ultra violet light, the varnish surface was. still; tacky. The. varnished leather was then stretched on a frame and introduced into a chamber havinga volume of 9 cubicmeters. The chamber had aheating plate and evaporating dish ofporcelain and a glass tube, as. described: inExample' 1. 30 grams ofa mixture of 1 methyl-26 phenylene diisoc-yanate and l methyl-2-4 phenylene diisocyanate were: introduced by means of the glass tube into the porcelain dish, drop by drop. T he'heating plate evaporated. thezisocyanateand the vapors were distributed by a fan positioned in the lower portion of the chamber. The temperature in the chamber was about 20 C. Through an observation window itwas possible to observe the evaporation process and thus to regulate the introduction of the isocyanate.

After the treatment, the leather showed an excellent, uniform varnish coating which was not at all tacky and which couldbe sharply creased without cracking. The leather was placed in a pile with the varnished layers against each other. After being stored for about half a year, nov tacking or adhesion" together of the varnished layers occurred.

Example 3 20' skins having. a total area of 270 square feet were varnish coated with linseed oil varnish and oven dried, in the-manner described in Example 1. After the oven drying, thev skins still had a tacky varnish coating. The skins were subjected to 30 minutes of ultra violet light and then placed in the treatment, chamber described in Ex,- ample 2. In the treatment chamber, 50 grams of 1 methyl- 2-6 phenylene diisocyanate were evaporated and the skins were subjected to the vapors for 60. minutes. The temperature was about 20 C. After the treatment, the varnish surface on the skins was no longer tacky. Even when stored for considerable periods of time with the varnish layers being placed one against the other, no adhesion together was observe. The skins could be creased sharply without cracking the varnish coating.

Example 4 20 skins having a total area of 2-70 square feet, after having been varnish coated and ovendried as describedin Example 1, were subjected for 105 minutes to ultra violet irradiation and thereupon placed in the treatment chamber described in Example 2. 30.- grams of a mixture of 1 methyl-24 phenylene' diisocyanate and 1 methyl-2-6 phenylene diisocyanate were evaporated in: the chamber and the skins were exposed to the vapors for 30 minutes. The temperature inthe chamber was. about 49 C. After the treatmenhthe skins-showed notackiness on their varnished surface and, when placed in a pile with the varnish layers: facing each other and stored six months, no. adhering or tacking together occurred.

I claim:

1. in the process for the production of patent leather, in which. leather is. coated with linseed oil varnish and oven dried to tacky dryness, the improvement which comprises contacting the dried tacky varnish coating with vapors of. a diisocyanate capable of. reacting with and hardening, the coating until only substantially the surface of the coating is hardened to non-tacky dryness.

2. Improvement according to claim- 1, in which said diisocyanate is a member selected from the group consisting of hexamethylene diisocyanate, 1 methyl-2-4 phenylene diisocyanate, l methyl-2-6 phenylene diisocyanate and mixtures thereof.

3. Improvement according. to claim 1, in which said contacting with saidvaporsis. effected. for about 3-5 hours.

4. Improvement according to claim 1, which includes prior to said contacting irradiating the dried tacky varnish coating with ultra violet light of an intensity for a period of time sufiicient to effect partial hardening of the tacky surface of said coating, but insufficient to completely harden the tacky surface of said coating.

5. Improvement according to claim 4, in which said irradiating is effected for a period of time of about onehalf to one hour.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN THE PROCESS FOR THE PRODUCTION OF PATENT LEATHER, IN WHICH LEATHER IS COATED WITH LINSEED OIL VARNISH AND OVEN DRIED TO TACKY DRYNESS, THE IMPROVEMENT WHICH COMPRISES CONTACTING THE DRIED TACKY VARNISH COATING WITH VAPORS OF A DIISOCYANATE CAPABLE OF REACHING WITH AND HARDENING THE COATING UNTIL ONLY SUBSTANTIALLY THE SURFACE OF THE COATING IS HARDENED TO NON-TACKY DRYNESS. 